[libc] [llvm] [libc][math] Fix powf exact cases, improve its performance, and add code-size optimized float-only version. (PR #226588)

[via llvm-commits]

zimmermann6 wrote:

I still get a spurious overflow:
```
zimmerma at biscotte:~/svn/core-math$ CORE_MATH_CHECK_STD=true LIBM=libllvmlibc.a ./check.sh powf
Running worst cases check in --rndn mode...
Spurious overflow exception for x,y=0x1.fffffep+64,0x1p-149 (z=0x1p+0)
```

https://github.com/llvm/llvm-project/pull/226588